Research On The Synthesis And Electrochemical Properties Of Li₃V₂（PO₄）₃ Cathode Material For Lithium-ion Batteries

The olivine-type lithium vanadium phosphate is considered as a promising cathode material for electric vehicle power after LiFePO4, due to its high specific capacity and high operating voltage. In order to solve the problems of low electronic conductivity and difficult synthesis of olivine-type Li3V2(PO4)3, in this paper, solid-phase method and hydrothermal method were used for materials preparation for improving the electrochemical performance. The structure, morphology and electrochemical properties of the samples were characterized by TG, XRD, SEM and electrochemical tests.Li3V2(PO4)3/C composite material was prepared via a two-stage solid-phase method. Factors affecting materials performance were investigated, such as final sintering temperature, carbon ratio, carbon sources, and lithium excess. The optimum conditions to synthesize Li3V2(PO4)3/C were determined. The results showed that the initial discharge capacity and capacity retention ratio were best with the carbon ratio of 20 wt %. The electrochemical performance of cathode materials by sucrose as a carbon source was much better than the one’s using carbon nanotubes as carbon source. The electrochemical performance of the materials was best at 750℃, which the initial specific capacity displayed 136.8mAh/g. After 20 cycles, discharge capacity still kept 120mAh/g, giving a capacity retention rate of 87.7%. Lithium excess can increase the initial discharge capacity of materials. Especially at lithium excess of 10%, the capacity of materials was improved significantly, the initial discharge capacity was 157.7mAh/g.Li3V2(PO4)3/C composite material prepared by hydrothermal method were investigated. The effects of different annealing temperatures, different hydrothermal temperatures and conductive agents on the performance of materials were discussed. The results indicated that the best first discharge capacity and cyclic performance of the material were given when hydrothermal temperature was 120℃. SEM tests howed that the higher the hydrothermal temperature was, the bigger the particle size of the material was. When the annealing temperature was 800℃, the capacity retention ratio of the materials was best, which was up to 90%. The influences of different conductive agents(carbon nanotubes and acetylene black) on the electrochemical performance of the materials were investigated.The results showed that the addition of carbon nanotubes for improving cycling performance of the materials was more significant, but there was no significant improvement in capacity of the materials.Mg2+doping was used to optimize the electrochemical performance of Li3V2(PO4)3/C. The effects of different doping amounts on the performance of the Li3V2-xMgx(PO4)3/C cathode materials were investigated by XRD, SEM, and electrochemical tests. There was no impurity peak of magnesium ion appeared when doping amount x was less than 0.3. However, the particle size of materials was increased with the increasing of doping concentration. The first discharge capacity of the material was improved with Mg2+ doping amount of 0.1. The electrochemical performance of the material gradually deteriorated accompanied with increasing of doping amount, the initial discharge capacity was 122mAh/g. Mg2+ doping can improve the ionic conductivity and ion mobility of Li3V2(PO4)3/C, so the specific capacity of the material were significantly improved.